1. Field of the Invention
The present invention relates to a document reading apparatus operating in a monochromatic document reading mode and a color document reading mode and configured to execute variable magnification processing on a read document image.
2. Description of the Related Art
Recently, Japanese Patent Application Laid-Open No. 2002-247290 has discussed a document reading apparatus having a four-line sensor, which includes a monochromatic document reading line sensor in addition to a conventional color document reading three-line sensor. The document reading apparatus discussed in Japanese Patent Application Laid-Open No. 2002-247290 is intended to both read a monochromatic document at a high speed and read a color document with a high image quality.
The above-described type of document reading apparatus outputs an image signal from the monochromatic document reading line sensor by using two channels including an odd-numbered pixel output channel and an even-numbered pixel output channel. With this configuration, the above-described type of conventional document reading apparatus can increase the reading productivity of the monochromatic document reading line sensor twice as high as that of the line sensor for color document reading. Accordingly, the conventional document reading apparatus described above can read a large number of monochromatic documents in a short period of time.
In a document reading apparatus capable of reading monochromatic documents at a high speed, the following problems may arise. The difference of the levels of productivity or the start-up timings may appear between the document reading apparatus and an image forming apparatus in transmitting image data from the document reading apparatus which executes high-speed document reading, to an image forming apparatus such as a printer.
If the productivity of the document reading apparatus is higher than that of the image forming apparatus or if the document reading start timing of the document reading apparatus is earlier than that of the image forming apparatus, then it becomes necessary to temporarily store the image data of the read document on a memory.
In reading monochromatic documents at a high speed and temporarily storing the read image data on a memory, it is necessary to temporarily store image data of a larger number of pages than in the case of reading color documents. In order to address this, Japanese Patent Application Laid-Open No. 2006-086629 discusses the following method. More specifically, the method discussed in Japanese Patent Application Laid-Open No. 2006-086629 binarizes multivalued image data of monochromatic documents and stores the binary data on a memory. On the other hand, the multivalued image data of color documents is stored as it is (without binarizing the same) on a memory.
Accordingly, the method discussed in Japanese Patent Application Laid-Open No. 2006-086629 can store image data of a large number of pages by binarizing and storing image data of monochromatic documents, which primarily include text data, on a memory having a limited capacity.
However, in executing magnification or reduction (hereinafter simply referred to as “variable magnification”) on the monochromatic images, if the binary image stored on the memory is digitally variable-magnified by interpolating or thinning out the same, then image degradation, which includes a phenomenon of step-like difference on an edge of the image or image blur, may occur.
In order to suppress image degradation and address the above-described problem, it may be effective to execute variable magnification scanning, in which variable magnification in the sub scanning direction is executed by reading a document while conveying the document at a speed appropriate for the variable magnification ratio.
However, if the variable magnification scanning is executed in reading a color document, color misregistration may occur in the image due to vibration that may occur in driving the optical system at a low speed during enlargement processing.